We and others have reported the isolation of a clone encoding for the CCAAT/Enhancer Binding Protein (C/EBP)beta (LAP, NF-IL6, IL6-DBP), a liver-enriched activator protein that confers liver-specific gene expression. This proposal will focus on the signal transduction pathways that modulate hepatocyte proliferation and survival through site-specific phosphorylations of C/EBPbeta. We have found that transforming growth factor (TGF)-alpha and phorbol esters/protein kinaseC (PKC)-alpha mediate their effects on hepatocyte proliferation, through the activation of ribosomal S6-kinase (RSK)- 2 and phosphorylation of C/EBPalpha on its activation domain. We have established that C/EBPbeta affects the G1/S phase transition of the cell cycle. Moreover, we found that this phosphorylated C/EBPbeta plays a major role in cell survival by binding to procaspase 8 and preventing its self-cleavage and activation. Our recent discovery identifies a novel nontranscriptional role of C/EBPbetaPThr217as the first mammalian protein acting as an inhibitor of caspases. As expected, we have found that the non-phosphorylatable C/EBPbetaAla217mutant prevents hepatocyte proliferation induced by the TGFalpha/ERK/MAPK/RSK signaling cascade, while the phosphorylation mimic C/EBPbetaGlu217 mutant blocks hepatocyte apoptosis initiated by death receptors or proteasome inhibitors (FAS/caspase 8 pathway). The specific aims of this proposal are to assess: 1. The regulation of the cell cycle by C/EBPbeta in hepatocytes. 2. The regulation of hepatocyte proliferation and tumorigenesis in C/EBbetAla217transgenic mice. 3. The role of C/EBPbeta phosphorylation on hepatocyte survival. 4. The modulation of hepatocyte cell survival in C/EBPbeta Glu217 transgenic mice. 5. The effects of C/EBPbetaGlu217peptides on hepatocyte survival. 6. The role of C/EBPbeta phosphorylation on the development of hepatocellular carcinoma in patients. These studies are relevant to the hepatocyte proliferation and survival characteristic of many physiological and pathological conditions, including hepatocellular injury, liver regeneration and hepatocellular carcinoma.